1. Field of the Invention
The present invention relates to a catalyst degradation diagnosing apparatus for an air fuel ratio control system, and more specifically to a catalyst degradation diagnosing apparatus which can reliably diagnose catalyst degradation without being subjected to the influence of engine operating conditions.
2. Description of the Prior Art
An apparatus for diagnosing catalyst degradation in a so-called double O.sub.2 sensor system (such that two oxygen sensors are disposed on the upstream (front) side and downstream (rear) side of a catalytic converter, respectively) is disclosed in Japanese Published Unexamined (Kokai) Patent Appli. No. 61-286550.
FIG. 1A is a routine for calculating an air fuel ratio feedback correction coefficient .alpha. on the basis of an output voltage signal VFO of the front O.sub.2 sensor, which is executed at predetermined time intervals. In more detail, control first checks whether air fuel ratio feedback condition F/B by the front O.sub.2 sensor FO.sub.2 is established (in step S1). If NO, control proceeds to step S9 to fix the correction coefficient .alpha.. The feedback condition is not established when the coolant temperature Tw is below a predetermined value or the output voltage signal of the front O.sub.2 sensor is not yet inverted once (because the amount of fuel is increasing at engine start or immediately after engine start or during engine warming up) or no fuel exists. However, the air fuel ratio feedback condition is usually established except the above conditions.
If YES, control compares the front O.sub.2 sensor output voltage VFO with a slice level SL.sub.F corresponding a stoichiometric mixture ratio (a theoretical air fuel ratio) and determines that the air fuel ratio is on the rich side if VFO.gtoreq.SL.sub.F and on the lean side if VFO&lt;SL.sub.F (in step S2). Further, control checks again whether the air fuel ratio is rich at the proceeding check (in steps S3 and S4). Therefore, four cases can be obtained on the basis of the check result (in steps S2 to S4), and the respective air fuel ratio feedback correction coefficients .alpha. are calculated according to the check result (in steps S5 to S8) as follows:
(1) In the case of steps S2.fwdarw.S3.fwdarw.S5, control determines that the air fuel ratio has just been inverted from the lean side to the rich side, and subtracts a proportional value P.sub.R (where the suffix R denotes the rich side) from the current air fuel ratio feedback correction coefficient .alpha. as (.alpha.=.alpha.-P.sub.R), so that the air fuel ratio is stepwise returned to the lean side.
(2) In the case of steps S2.fwdarw.S3.fwdarw.S6, control determines that the ratio is still rich, and subtract an integration value I.sub.R from the current coefficient .alpha. as (.alpha.=.alpha.-I.sub.R), so that the air fuel ratio is gradually returned to the lean side.
(3) In the case of steps S2.fwdarw.S4.fwdarw.S7, control determines that the ratio has just been inverted from the rich side to the lean side and add a proportional value P.sub.L (where the suffix L denotes the lean side) to the current coefficient .alpha. as (.alpha.=.alpha.+P.sub.L), so that the air fuel ratio is similarly stepwise returned to the rich side.
(4) In the case of steps S2.fwdarw.S4.fwdarw.S8 control determines that the ratio is still lean and add an integration value I.sub.L to the current coefficient .alpha. as (.alpha.=.alpha.+I.sub.L), so that the air fuel ratio is gradually returned to the rich side. FIG. 1E-1 shows a wareform of the front O.sub.2 sensor output voltage VFO and FIG. 1E-2 shows a wareform of the air fuel ratio correction coefficient .alpha. both with the lapse of time, by way of example.
FIG. 1B is a routine for further correcting the air fuel ratio feedback correction coefficient .alpha. already corrected on the basis of the front O.sub.2 sensor output voltage signal VFO (as shown in FIG. 1A), additionally on the basis of the rear O.sub.2 sensor output voltage signal VRO, which is also executed at predetermined time intervals. In more detail, control first checks whether air fuel ratio feedback condition F/B by the rear O.sub.2 sensor RO.sub.2 is established (in step S11). If YES, control proceeds to step S12 and compares the rear O.sub.2 sensor output voltage VRO with a slice level SL.sub.R corresponding to a stoichiometric mixture ratio and determines that the air fuel ratio is on the lean side if VRO&lt;SL.sub.R, and proceeds to steps S13 and S14. However, if VRO.gtoreq.SL.sub.R, control determines that the air fuel ratio is on the rich side and proceeds to steps S15 and S16.
In step S13, a constant value .DELTA.P.sub.L is further added to the proportional value P.sub.L as (P.sub.L =P.sub.L +.DELTA.P.sub.L); and in step S14, a constant value .DELTA.P.sub.R is further subtracted from the proportional value P.sub.R as (P.sub.R =P.sub.R -.DELTA.P.sub.R), so that the air fuel ratio is shifted as a whole to the rich side. In the same way, in step S15, a constant value .DELTA.P.sub.L is subtracted from the proportional value P.sub.L as (P.sub.L =P.sub.L -.DELTA.P.sub.L); and in step S16, a constant value .DELTA.P.sub.R is added to the proportional value P.sub.R as (P.sub.R =P.sub.R +.DELTA.P.sub.L), so that the air fuel ratio is shifted to the lean side. As described above, the precision of the air fuel ratio feedback control on the basis of the front O.sub.2 sensor output voltage can be improved by the control of the correction coefficient .alpha. corrected on the basis of the rear O.sub. 2 sensor output voltage VRO as shown in FIG. 1B.
FIG. 1C shows a routine for calculating a fuel injection pulse width Ti, which is executed at predetermined crank angular intervals.
Control calculates a basic fuel injection pulse width T.sub.p =K.multidot.Qa/Ne (where K denotes a constant) on the basis of an intake air amount Qa and an engine speed Ne and with reference to a map (in step S21). Control calculates an addition Co of 1 and various correction coefficients (e.g. coolant temperature increase correction coefficients K.sub.TW) (in step S22), and determines a fuel injection pulse width Ti to be outputted to a fuel injector in accordance with the following expression (in step S23): EQU Ti=Tp.multidot.Co.multidot..alpha.+Ts
where Ts denotes the ineffective pulse width. Control sets the determined Ti (in step S24).
FIG. 1D shows a routine for diagnosing catalyst degradation. Control first checks whether a catalyst degradation diagnosis flag is set (in step S31). If NO, control resets the counter value to zero (in step S39). If YES, control compares the rear O.sub.2 sensor output voltage VRO with a slice level SL.sub.R corresponding to a stoichiometric mixture (theoretical air fuel) ratio and determines that the air fuel ratio is on the rich side if VRO.gtoreq.SL.sub.R and on the lean side if VFO&lt;SL.sub.F (in step S32). Further, control checks again whether the air fuel ratio is rich at the proceeding check (in steps S33 and S34). The counter value C is incremented by one in steps 35 and 36 on the basis of the check result (in steps S32 to S34). This counter value represents the number of inversions of the rear O2 sensor output voltage VRO across the slice level SL.sub.R.
Control compares the counter value C with a predetermined value (in step S37) and determines that the catalyst has been degraded if C exceeds the predetermined value, and inhibits the feedback coefficient .alpha. from being corrected by the rear O.sub.2 sensor (in step S38).
However, the prior-art apparatus for diagnosing catalyst degradation as described above involves the following drawbacks. Even in the feedback control by the front O.sub.2 sensor disposed on the upstream side of the catalytic converter, since the controlled air fuel ratio inevitably fluctuates slightly, when the air fuel ratio fluctuates under the condition that the output voltage VRO of the rear O.sub.2 sensor disposed on the downstream side of the catalytic converter is controlled near the slice level SL.sub.R, the rear O.sub.2 sensor output voltage VRO also fluctuates due to the fluctuations in the air fuel ratio. For instance, in the case of the new catalyst, the rear O.sub.2 sensor output VRO fluctuates slightly near the slice level SL.sub.R, as shown on the left side in FIG. 9A. In the case of the used catalyst, however, the rear O.sub.2 sensor output VRO fluctuates violently as shown on the right side in FIG. 9A.
Therefore, if the slice level SL.sub.R (shown at the step S12 in FIG. 1B) for feedback controlling the air fuel ratio on the basis of the rear O.sub.2 sensor output is the same as the slice level SL.sub.R (shown at the step S32 in FIG. 1D) for diagnosing the catalyst degradation, since the fluctuation period of the new catalyst is the same as that of the old catalyst and therefore the number of inversions of the rear O.sub.2 sensor output level across the slice level SL.sub.R is the same in both the new and used catalyst, it is impossible to detect the difference in catalyst degradation between the new catalyst and the used catalyst, thus it has been difficult to precisely diagnose the catalyst degradation.
To overcome the above-mentioned problem, it is possible to discriminate the used catalyst from the new catalyst or vice versa, by determining the slice level for feedback controlling the air fuel ratio and that for diagnosing the catalyst degradation separately as shown in FIG. 9B, in such a way that a rich discriminating slice level RSLH2 for diagnosing the catalyst degradation is set higher than a rich discriminating slice level RSLH1 for feedback controlling air fuel ratio and further a lean discriminating slice level RSLL2 for diagnosing the catalyst degradation is set lower than a lean discriminating slice level RSLL1 for feedback controlling air fuel ratio. This is because the inversion of the rear O.sub.2 sensor output level across the slice levels RSLH2 and RSLL2 for diagnosing the catalyst degradation can be detected only when the catalyst has been degraded.
In this method, however, since the amplitude and the period of the fluctuation waveform of the rear O.sub.2 sensor output VRO shown in FIGS. 9A and 9B vary according to engine operating conditions, when the slice levels RSLH2 and RSLL2 for diagnosing the catalyst degradation are fixedly determined, the rear O.sub.2 sensor output VRO changes in the same catalyst beyond or below the slice level RSLH2 or RSLL2, respectively according to the engine operating conditions, thus resulting in diagnosis error.